1. Field of the Invention
The present invention relates to semiconductor light-emitting devices. “Semiconductor light-emitting device” herein refers to a device formed by placing, on a mount member, a semiconductor light-emitting device chip such as semiconductor laser chip and LED chip and integrating the chip thereon. For example, a semiconductor light-emitting device including a semiconductor laser chip placed on a mount member is herein referred to as “semiconductor laser device.”
“Mount member” herein refers to a component on which a semiconductor light-emitting device chip is directly mounted. For example, “mount member” refers to a submount used for the semiconductor light-emitting device. If the chip is mounted, without submount, directly on a supporting base, i.e., stem, frame or package, mount member refers to the stem, frame or package.
2. Description of the Background Art
GaN-based semiconductor is a material used for realizing a semiconductor light-emitting device chip for the region ranging from violet to green in the spectrum, and accordingly attracting attention. In particular, a desired semiconductor laser is the one using GaN-based semiconductor for emitting radiation of shorter wavelengths than those which have been achieved. In order to use GaN-based semiconductor in the semiconductor laser chip, sapphire which is insulator has been used as a substrate of the semiconductor laser chip. However, in recent years, a semiconductor laser chip using GaN-based semiconductor therein and being mounted on a GaN substrate instead of the sapphire substrate has also been studied, the semiconductor chip having a structure including a stack formed of an n-type semiconductor layer, an active layer, a p-type semiconductor layer and an electrode for example that are successively formed on the GaN substrate.
Here, a semiconductor laser device includes a semiconductor laser chip and a supporting base on which the chip is mounted and is capable of emitting a laser beam in a desired direction. The semiconductor laser device in operation is required to efficiently transfer and liberate heat in order to prevent deterioration of its characteristics that is caused by increase in temperature of its light-emitting component. Then, it is essential to mount the semiconductor laser chip on the supporting base so that a satisfactory thermal conductivity is achieved. According to one method for mounting the chip with a satisfactory thermal conductivity, a component called submount is provided between the supporting base and the semiconductor laser chip. The chip may be mounted “junction-down” on the submount. Specifically, the semiconductor laser chip has a substrate on one side and a stack on the other side, the stack side is opposed to the submount and the chip and the submount are coupled by die bonding. The submount is then coupled to the supporting base by die bonding for example.
Japanese Patent Laying-Open No. 2000-58965 (hereinafter “document 1”) discloses an example of the semiconductor laser device employing the junction-down mounting method and an example thereof which can employ the junction-down mounting method.
The junction-down method is described specifically with reference to FIGS. 8 and 9 exemplarily showing conventional semiconductor laser devices to which the idea of document 1 is applied. It is noted that those components disclosed in document 1 that are not essential for the description here are simplified in the structures shown in the drawings.
FIG. 8 shows the semiconductor laser device employing the junction-down method. A semiconductor laser chip 130 includes a substrate 101 having a main surface on which a stack 102 including an active layer 106 is formed, and semiconductor laser chip 130 is mounted on a holding member 140 formed of SiC. Specifically, semiconductor laser chip 130 is mounted by being placed on holding member 140 with stack 102 opposite holding member 140, and solder 112 is used for connecting a p-type electrode 103 and an n-type electrode 104 respectively to metal films 141 and 142 on the top surface of holding member 140. SiC is used for holding member 140 since SiC is “insulator” and “excellent in thermal conductivity” as described in paragraph 0029 of document 1. Holding member 140 should serve to liberate heat and thus such a material having an excellent thermal conductivity is naturally selected. In stead of SiC, BN, AlN or diamond for example may be used as these materials are insulator and excellent in thermal conductivity as described in paragraph 0051 of document 1.
Here, semiconductor laser chip 130 is mounted on holding member 140 which may actually be regarded as a submount, as a component of the semiconductor laser device. The material for the component is selected on the same basis regardless of naming.
FIG. 9 shows a semiconductor laser chip 230 including a conductive substrate 201 formed of GaN for example having the top surface on which a stack 202 including an active layer 206 is formed. A p-type electrode 203 is formed on the top surface of stack 202 and an n-type electrode 204 is formed on the bottom surface of substrate 201. This semiconductor laser chip 230 can also be mounted “junction-down” by die bonding.
The die bonding follows a procedure as described below. Solder is applied in advance to the top surface of a submount, holding member or the like on which a semiconductor laser chip is to be mounted by die bonding. The solder is heated to at least its melting point, and the semiconductor laser chip aligned at a predetermined position is pressed against the melted solder by means of a collet. Then, the solder is cooled to solidify. In this way, the semiconductor laser chip and submount are coupled with a good thermal conductivity.
The inventors of the present invention have found that, if a semiconductor laser chip including a nitride-based compound semiconductor substrate and a nitride-based compound semiconductor which is formed on a main surface of the substrate is mounted junction-down on a submount for example, some characteristics of a resultant semiconductor light-emitting device could deteriorate. The deterioration here is increase of threshold current Ith for lasing. Threshold current Ith is desirably as low as possible.
FIG. 10 exemplarily shows a characteristic deterioration caused by the junction-down mounting. In FIG. 10, Ith before mounting is indicated by (a) and Ith after mounting is indicated by (b).